1. Field of the Invention
This invention relates to the detection of vibrations of a target and more particularly to the use of micro-doppler technology for detecting vibrations in very small objects.
2. Description of the Related Art
Research on the use of lasers to hold and manipulate living cells using focused laser beams has advanced rapidly in recent years and today laser xe2x80x9ctweezersxe2x80x9d and xe2x80x9cscissorsxe2x80x9d are significant tools in the study of living cells. This technology is now also being used in genetic engineering as well, where chromosomes are held and cut. Another application has been in fertility research where egg cell surfaces are prepared to facilitate fertilization. This technology is well described in a Scientific American article (August 1998) entitled Laser Scissors and Tweezers by Dr. Michael Berns of the University of California at Irvine. As described in the article, laser beams are focused to volumes as small as 0.25 micrometers, which permits individual organelles (e.g., ribosomes or mitochondria) to be held and manipulated. However, the capability to measure the vibrational spectrum of cellular structures simply does not exist at present.
The detection of target vibrations for the purpose of non-cooperative classification by coherent micro-doppler ladar is known. For example, U.S. Pat. No. 5,847,817, issued to Zediker et al, discloses a method for extending the effective range and sensitivity of a micro-doppler ladar system by increasing its effective coherence length. U.S. Pat. No. 5,847,816, also issued to Zediker et al, discloses a micro-doppler ladar system constructed primarily with fiber optic elements. U.S. Pat. No. 5,867,257, issued to Rice et al, discloses a battlefield personal threat detection system for identifying and analyzing vibrations corresponding to an immutable characteristic of a target of interest such as the heartbeat of an enemy soldier or the respiration process of the soldier. However, micro-doppler technology has not been extended for use with very small objects, such as cellular structures.
The present invention is a micro-doppler microscope for providing an indication of the vibration of a target. An input laser beam is received by a beamsplitter that divides the input laser beam into a transmitted portion and a reflected portion. A first focusing optics focuses the transmitted portion onto a target, the target scattering a scattered portion thereof. This scattered portion is directed back through the first focusing optics, and reflected off the beamsplitter. Second focusing optics is positioned to receive the scattered portion from the beamsplitter and for focusing the scattered portion. A planar mirror is positioned to reflect the reflected portion of the input laser beam back through the beamsplitter and through the second focusing optics so that it is focused. A fast photodetector receives the focused scattered portion from the second focusing optics and the focused reflected portion from the second focusing optics. Both focused portions are focused on the same spot on the fast photodetector. Coherent mixing of the focused portions occurs at the fast photodetector, the frequency modulation of the scattered portion thereby providing an indication of the vibration of the target. The indication is provided as an output signal from the fast photodetector. This output signal may be directed to signal processing electronics where it may presented, for example, on a display as an indication of motion. Alternatively, the signal may undergo spectrographic analysis.